The present invention relates to the communications satellite art. Specifically, an improved bearing and power transfer assembly for coupling a spun and despun shelf together in a spinning satellite is described.
Satellite transponders for permitting communications between two earth located stations have been in use for many years. Presently such satellites are located in geosynchronous orbits whereby their position with respect to the earth's surface remains relatively stationary to permit radio signals to be transmitted and received from stationary earth stations.
The satellite transponder, once in orbit, erects in response to a ground command a directional transmit-receive antenna. In order to receive and transmit through the transmit-receive antenna, the antenna must be maintained in an earthward-looking direction. The difficulty in maintaining the required antenna positioning results from a directional spin which the satellite, once in orbit, experiences at a rate of approximately one revolution per second. To avoid this difficulty, the antenna is despun by rotating the supporting shelf in a counter direction at the spin rate to achieve a relatively constant earthward look angle for the transmit-receive antenna.
The interface between the spun portion of the satellite and the despun shelf is provided by a bearing and power transfer assembly. The spinning or spun portion of the satellite contains batteries, apogee kick motors and earth sensors and decoders. The despun shelf carries the transmit antenna, telemetry receivers and TWT amplifiers heat sinked to the exterior of the despun shelf. The bearing provides a ball bearing interface between spun and despun shelves, and provides through a motor the requisite rotational torque to despin the despun shelf. The bearing also carries a plurality of brush-slip ring assemblies to carry power and telemetry signals between shelves.
The bearing and power transfer assembly has been identified in a report entitled "Summary Report of a Study of Increased Reliability and Lifetimes for Future Communcations Satellites", under a contract to the European Space Agency, as a major source of satellite failure. Once the despin bearing structure fails, the entire satellite is rendered useless for further communication.
The present invention seeks to provide a back-up bearing structure which can be activated by ground commands in the event of a failure of the main bearing and power transfer assembly, thus restoring the satellite to use.